212 research outputs found
Does mass accretion lead to field decay in neutron stars
The recent discovery of cyclotron lines from gamma-ray bursts indicates that the strong magnetic fields of isolated neutron stars might not decay. The possible inverse correlation between the strength of the magnetic field and the mass accreted by the neutron star suggests that mass accretion itself may lead to the decay of the magnetic field. The spin and magnetic field evolution of the neutron star was calculated under the hypothesis of the accretion-induced field decay. It is shown that the calculated results are consistent with the observations of binary and millisecond radio pulsars
Quasiperiodic oscillations in bright galactic-bulge X-ray sources
Quasiperiodic oscillations with frequencies in the range 5-50 Hz have recently been discovered in X-rays from two bright galactic-bulge sources and Sco X-1. These sources are weakly magnetic neutron stars accreting from disks which the plasma is clumped. The interaction of the magnetosphere with clumps in the inner disk causes oscillations in the X-ray flux with many of the properties observed
X-Ray and Gamma-Ray Emission from the PSR 1259-63 / Be Star System
PSR 1259-63 is a radio pulsar orbiting a Be star in a highly eccentric orbit.
Soft and hard X-rays are observed from this binary system. We apply the shock
powered emission model to this system. The collision of the pulsar and Be star
winds forms a shock, which accelerates electrons and positrons to the
relativistic energies. We derive the energy distribution of relativistic
electrons and positrons as a function of the distance from the shock in the
pulsar nebula. We calculate the X-rays and -rays emitted from the
relativistic electrons and positrons in the nebula at various orbital phases,
taking into account the Klein-Nishina effect fully. The shock powered emission
model can explain the observed X-ray properties approximately. We obtain from
the comparison with observations that a fraction of of the pulsar
spin-down luminosity should be transformed into the relativistic electrons and
positrons. We find that the magnetization parameter of the pulsar wind, the
ratio of the Poynting flux to the kinetic energy flux, is
immediately upstream of the termination shock of the pulsar wind, and may
decrease with distance from the pulsar. We predict the flux of 10 MeV - 100 GeV
-rays which may be nearly equal to the detection threshold in the
future projects.Comment: 18 pages, 9 figures, accepted for publication in PAS
THERMAL RADIATION FROM MAGNETIZED NEUTRON STARS: A look at the Surface of a Neutron Star.
Surface thermal emission has been detected by ROSAT from four nearby young
neutron stars. Assuming black body emission, the significant pulsations of the
observed light curves can be interpreted as due to large surface temperature
differences produced by the effect of the crustal magnetic field on the flow of
heat from the hot interior toward the cooler surface. However, the energy
dependence of the modulation observed in Geminga is incompatible with blackbody
emission: this effect will give us a strong constraint on models of the neutron
star surface.Comment: 10 pages. tar-compressed and uuencoded postcript file. talk given at
the `Jubilee Gamow Seminar', St. Petersburg, Sept. 1994
The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE
Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed
156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this
source. We investigated the fast-time variability as a function of position on
the Z track. During these observations, Sco X-2 traced out the most extensive Z
track ever reported from this object, making this the most comprehensive study
thus far. We found the broad peaked flaring branch noise that is typical of Sco
X-2, with a centroid frequency in the range 3.3--5.8 Hz. We also discovered low
frequency noise, and a new peaked noise feature, with centroid frequencies in
the range 5.4--7.6 Hz and 11--54 Hz, respectively. We discuss the phenomenology
of these features, their relationship with the power spectral components found
in other low-mass X-ray binaries, and the implications for current models. In
particular, the low frequency noise we observed was strongest at intermediate
energies, in contrast to the low frequency noise seen in other Z sources. We
also detected very low frequency noise, and have calculated complex cross
spectra between intensity and hardness. We found that the very low frequency
noise is not entirely due to motion along the Z track.Comment: 17 pages, 9 figures, minor improvements, accepted for publication in
MNRA
Thermal Evolution of Compact Stars
A collection of modern, field-theoretical equations of state is applied to
the investigation of cooling properties of compact stars. These comprise
neutron stars as well as hypothetical strange matter stars, made up of
absolutely stable 3-flavor strange quark matter. Various uncertainties in the
behavior of matter at supernuclear densities, e.g., hyperonic degrees of
freedom, behavior of coupling strengths in matter, pion and meson condensation,
superfluidity, transition to quark matter, absolute stability of strange quark
matter, and last but not least the many-body technique itself are tested
against the body of observed cooling data.Comment: 41 pages, revised versio
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
Testing the Accretion-induced Field-decay and Spin-up Model for Recycled Pulsars
Millisecond radio pulsars have long been proposed to form from a spin-up
recycling process in a binary system. In this paper we demonstrate that the
accretion-induced field-decay and spin-up model for recycled pulsars can indeed
produce those millisecond pulsars with relatively weak magnetic fields of
G and short spin periods of a few milliseconds. Our results also
suggest that the value of the currently observed highest spin frequency of
millisecond pulsars may simply be constrained by the amount of mass available
for accretion.Comment: 7 pages, 5 figures, accepted by A&
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